Water Management Reference
Demand by stage, the arithmetic for sizing, and how to schedule without guessing.
Crop water demand#
Banana has one of the highest and most consistent water demands of any tropical crop — the fruit is over 90% water and the canopy transpires heavily.
| Parameter | Value |
|---|---|
| Base ET₀ (tropical lowland) | 4–6 mm/day |
| Crop coefficient Kc, initial | 0.75 |
| Crop coefficient Kc, mid-season (full canopy) | 1.15–1.25 |
| Crop coefficient Kc, late season | 1.05–1.15 |
| Peak crop ETc | 6–9 mm/day |
| Whole-cycle requirement | 1,400–2,000 mm (up to 2,200 in hot, windy, low-humidity sites) |
| Per-plant daily use, mature | 40–90 L (up to 120 L on hot, windy days) |
| Water per kg of fruit | roughly 100–200 L including field losses |
By growth stage#
| Stage | Kc | ETc (mm/day) | Priority |
|---|---|---|---|
| Establishment (0–3 mo) | 0.75–0.90 | 3–5 | High — sets the stand |
| Vegetative (3–6 mo) | 1.00–1.15 | 5–7 | High |
| Flowering (−6 to +4 wk) | 1.20–1.25 | 7–9 | Critical |
| Filling | 1.15–1.25 | 6–9 | Critical |
| Pre-harvest | 1.05–1.15 | 4–6 | Moderate |
The sizing calculation#
Daily crop need (m³/day) = area (m²) × ETc (mm/day) ÷ 1,000
Required system flow (m³/h)
= daily need ÷ (planned irrigation hours/day)
÷ system efficiency (drip 0.85–0.90, micro-sprinkler 0.75–0.85, flood 0.50–0.60)
Worked example — 3 ha, peak 8 mm/day, 12 h/day, micro-sprinkler at 80%:
daily need = 30,000 m² × 8 ÷ 1,000 = 240 m³/day
flow = 240 ÷ 12 ÷ 0.80 = 25 m³/h (≈ 417 L/min)
+ 20% for flushing, leakage, unevenness = 30 m³/h design flowThen verify the dry-season source yield can sustain 30 m³/h continuously — a wet-season borehole result proves nothing about July.
Plant counts come from the density calculator; full system build instructions in Setting up irrigation.
System comparison#
| System | Application efficiency | Uniformity | Capital cost | Water use per tonne | Best suited to |
|---|---|---|---|---|---|
| Drip (2–8 L/h emitters) | 85–92% | Very high | Medium–High | Lowest | Water-scarce, sandy, sloping, fertigated blocks |
| Micro-sprinkler | 75–88% | High | Medium | Low | Most commercial banana; larger wetted bulb |
| Mini-sprinkler / rain gun | 65–80% | Moderate | Medium | Moderate | Large flat blocks; risk of runoff |
| Furrow / flood | 45–65% | Low to moderate | Lowest | Highest | Flat land, cheap water, no slope |
| Rainfed | n/a | n/a | None | n/a | 2,000+ mm well distributed |
Scheduling methods#
1. Soil moisture (recommended baseline)#
| Method | Set point | Notes |
|---|---|---|
| Tensiometer at 20 cm and 40 cm | Irrigate at −20 to −30 kPa at 20 cm; never exceed −50 kPa | Simple, visual, very reliable |
| Soil probe / auger | Cannot penetrate to 20 cm = irrigate | Cheapest useful tool; train staff to use it consistently |
| Capacitance / TDR sensor | Crop-specific threshold | Continuous data; needs calibration |
| Visual canopy | Midday flaccidity = act now | Late — use only as a backup signal |
2. Weather-based (ET)#
Irrigation depth (mm) = ET₀ × Kc × days since last irrigation − effective rainfall − soil moisture surplus
An ET station or a reference table plus a local weather reading gives a defensible schedule. Effective rainfall is usually counted as rainfall × 0.75, and only for events above ~5 mm.
3. Calendar (last resort)#
Fixed-interval scheduling wastes water when it rains and under-waters in a dry spell. Use only where no other monitoring is possible.
Best practice: combine methods — ET to set the depth, soil probe to verify it reached 30–40 cm, canopy observation as a sanity check.
Amount per irrigation#
| Goal | Depth to apply |
|---|---|
| Wet the full active root zone | 30–40 mm (30–40 L/m²) |
| Per plant, micro-sprinkler at 2 m² wetted area | ~60–80 L |
| Per plant, single drip emitter | 30–50 L, more often |
| Establishment | 10–15 L at planting, then 10–20 L per application for the first 3 weeks |
Do not irrigate short and often to a wet surface. That builds shallow roots that fail at the first dry week. Apply enough to wet 30–40 cm, then let the top few centimetres dry between applications.
Drainage#
| Threshold | Action |
|---|---|
| Water standing > 24 hours | Emergency drainage — bananas suffer measurable damage |
| Water table within 60 cm in the wet season | Raised beds, contour drains, or accept unsuitability |
| Infiltration < 10 mm/h | Investigate compaction, crusting or sodicity |
| Algal film / perpetual surface wetness | Over-irrigation or blocked drains |
Drainage is cheaper than replanting. Design it with the irrigation — both share the same field layout.
Water quality#
| Parameter | Good | Caution | Problem |
|---|---|---|---|
| pH | 6.0–7.5 | 5.5–6.0 or 7.5–8.5 | Affects nutrient availability and emitters |
| EC (dS/m) | < 0.8 | 0.8–1.5 | > 2.0 needs leaching and management |
| SAR | < 3 | 3–6 | > 6: sodicity risk, soil structure collapse |
| Bicarbonate (mg/L) | < 100 | 100–300 | > 300: Fe/Zn lock-up, emitter scale |
| Chloride (mg/L) | < 100 | 100–350 | > 700 risk of toxicity |
| Sodium | Low | Moderate | Salinity and structure risk |
| Iron / manganese | < 0.2 mg/L | 0.2–1.0 | Fe precipitates and blocks emitters |
| Sulfates | < 200 | 200–400 | Scale with calcium |
| Suspended solids | < 50 mg/L | 50–100 | Filtration required |
| Biological (faecal coliforms) | Potable / low | — | Risk if sprayed on fruit or contact |
Recording#
| Record | Why |
|---|---|
| Hours run per zone per day | Water accounting and cost |
| Water meter readings | Actual applied depth vs plan |
| Soil probe readings by block | Validates the schedule |
| Rainfall per event | Avoids unnecessary irrigation |
| Pump energy per hectare | Largest variable cost in many systems |
| Yield by block | Links water management to results |
Common problems#
| Problem | Cause | Fix |
|---|---|---|
| Dry patches in the block | Blocked laterals, pressure variation | Flush, clean filters, pressure-regulate |
| Uniform wilting despite irrigation | Root rot, salinity, or wet-but-airless soil | Inspect corm; check drainage and EC |
| Yellow lower leaves with wet soil | Waterlogging | Reduce frequency; open drains |
| Salt crust at wetted edge | Excess EC or insufficient leaching | Leach; improve water quality; mulch |
| Algae in tank and lines | Light + nutrients in the water | Shade tanks; treat; backwash |
| Emitters blocked with white scale | Hard water / bicarbonate | Acid flush per label |
| Emitters blocked with slime | Biofilm | Chlorine/permanganate treatment |
Next steps#
- Install it: Setting up irrigation.
- Pair with nutrition through fertigation: Nutrition.
- Compute plant counts and flow: Fertiliser calculator and Density calculator.
- Capture and store your own supply: Water harvesting.